The properties of bioregulatory proteins found in the near-term mouse placenta are under investigation. Included are growth factor (PlGF) proteins which induce or stimulate mitogenesis and colony growth in soft agar, and interferons (IFN-P1) which inhibit cell and viral proliferation. IFN-P1 is believed to be a unique interferon, and is distinguishable from the more classical alpha, beta, and gamma inteferons by serology and by the decay kinetics of its activity. It is, however, functionally similar to the classical interferons in many aspects including the induction of 2-5 A synthetase, and the inhibition of growth factor mediated mitogenesis. Mouse PlGF is unable to compete with raiolabeled-EGF for binding to cell receptors. Mouse PlGF readily absorbs at low ionic strength to agarose charged with Green A dye-ligand and is eluted by KC1. A growth factor resembling mouse PlGF is present in the human placenta at a concentration analogous to that of mouse PlGF.